1. Field of the Invention
The present invention relates to a light-receiving device, and more particularly, to a light-receiving device formed by using a group III nitride semiconductor.
2. Description of the Background Art
Solar-blind (which means that ultraviolet rays from sunlight are not sensed) ultraviolet sensors are used in fire detectors, for consumer use, and are used in various types of industrial furnaces, boilers for detecting accidental fire, internal combustion engines for monitoring combustion, measurement equipment and the like, for industrial use. For example, phototube-type ultraviolet sensors are widely used as the ultraviolet sensors for industrial purpose.
However, it is generally known that the phototube-type ultraviolet sensor is costly per se and its life is short. There is a possibility that an ultraviolet sensor which has a small size and long life may be realized at low cost if, in place of the above-mentioned conventional products, the ultraviolet sensor is formed of a semiconductor light-receiving device which is a solid-state element. As types of general semiconductor light-receiving devices, there are publicly known photo-resistive type (phototube type), Schottky diode type, P(I)N diode type, avalanche diode type and the like (for example, see S. M. Sze (trans. by Yasuo Nannichi, Mitsuo Kawabe and Fumio Hasegawa), “Semiconductor devices—Physics and Technology”, pp. 293-302).
Meanwhile, it is widely known that a group III nitride semiconductor has a direct-transition type wide bandgap and is capable of changing the bandgap through control of a mixed crystal composition, and thus can be used as a material for a light-emitting device and a light-receiving device whose light-emitting/receiving wavelength regions range from a visible region to an ultraviolet region. For example, a Schottky diode type group III nitride semiconductor light-receiving device (for example, see A. Osinsky, S. Gangopadhyay, B. W. Lim, M. Z. Anwar, M. A. Khan, D. V. Kuksenkov and H. Temkin, “Schottky barrier photodetectors based on AlGaN”, Applied Physics Letters, Vol. 72, No. 6, PP. 742-744) and a PN diode type group III nitride semiconductor light-receiving device (for example, see Japanese Patent Application Laid-Open No. 2000-101127) have been publicly known. In principle, it is possible to form an ultraviolet light-receiving device of each of the above-mentioned types using a group III nitride semiconductor.
In a case where use of the light-receiving device is spread out to an ultraviolet sensor, high light-receiving sensitivity is required as basic performance. That is, large photocurrent (output current when light is received) and small dark current (output current when light is shielded) are required. Specifically, it is desirable that, when incident light intensity is 10 μW, photosensitivity (S/N ratio) have five or more digits and the photocurrent be approximately 1 A/W or more.
Meanwhile, in a process of manufacturing a Schottky diode type semiconductor light-receiving device using a group III nitride semiconductor, a Schottky electrode composed of a metal material having a high work function, such as Pd, Pt and Ni, is formed by deposition on a semiconductor layer, in which heat treatment is not generally performed after the deposition. This is because, if the heat treatment is performed, Schottky characteristics are impaired and thus the dark current is increased in some cases. However, in a state where the metal electrode is merely deposited, electric contact between metal and the semiconductor layer is not sufficiently obtained, and accordingly, the dark current is increased or the metal film peels off in some cases.